In the hot early universe, like in an overcrowded dance hall, magnetic monopoles swirled — loner particles carrying only a north or only a south pole. In an ordinary magnet, poles always come in pairs, but here each one is on its own. Theories that unify all forces of nature predict that such particles should have been produced in vast numbers, yet today we don't see them.
The usual escape is cosmic inflation: space stretched faster than light and diluted the monopoles to near invisibility. The trouble is that inflation might have ended before the monopoles even appeared. Then they would fill the world again, and the puzzle remains. The authors of the new work propose a different path. After the first stage of cooling, the forces of nature briefly rearrange. Each monopole ends up tethered to its antimonopole by a long cosmic string — an ultra-thin tube of energy left over from the previous vacuum state.
This tether forces the pair to rapidly approach and annihilate, turning into radiation. Then symmetry is restored to the familiar Standard Model, and the world becomes as we know it. The restoration process itself may proceed in a jump, like water freezing into ice, but on the scale of the entire universe. If the jump is sharp enough, it shakes spacetime and gives birth to gravitational waves — ripples that sensitive antennas of future experiments could catch.
So invisible monopoles might reveal themselves not by light, but by the trembling of space itself. And if next-generation detectors pick up this signal, we'll get a message from an era when the universe was trillions of times hotter than now. Like the echo of a distant dance, frozen in gravitational ripples.
The magnetic monopole was first described by physicist Paul Dirac in 1931, and its existence would explain why electric charge always comes in multiples of the electron's charge.
In Robert Charles Wilson's novel Vortex, hypothetical monopoles are used to create portals between worlds.